A $10 MILLION bonus is being added to tonight's Mark Six jackpot in a bid to boost summer sales. It is the first time the Royal Hong Kong Jockey Club has added such a bonus during the summer. A spokesman denied that ticket sales were suffering. ''Sales fluctuate all the time depending on the size of the jackpot. This is just an incentive, like our Easter bonus or Chinese New Year snowball,'' he said. ''We will review the situation later to decide whether we should do this again.'' The bonus will be added to the first prize pool, already at $9.69 million. Sales of the $4 tickets had topped five million by 5 pm last night.